Known as the Brabant Killers, assailants firing pump-action shotguns killed 28 shoppers and wounded hundreds more in attacks on supermarkets and hostels during a string of robberies in the Brussels area between 1982 and 1985.
Teams of police began excavating parts of the Brussels-Charleroi canal close to the town of Ittre, 30 kilometres (18 miles) from the capital, after a group of young people informed officers they had discovered two firearms and a box of ammunition in the area, inscribed with the word "gendarmerie", the two prosecutors leading the investigation explained on Thursday.
The youths contacted police after watching a television programme about the murders on Tuesday.
"The investigation has been relaunched," said Ignacio de la Serna, attorney general of Mons.
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He said the ammunition found apparently dated from the 1980s but stressed: "I want to be very careful until we have an expert report in line with procedures".
The same portion of canal was inspected by police in the '80s when abandoned equipment used by the killers, including a bulletproof vest, was discovered in 1985.
The gang's equipment and ability in handling weapons raised suspicions at the time that there was a link to the gendarmerie, a former Belgian paramilitary police force.
Over the decades, investigators have considered various theories including that the group was part of an attempt by the far left or far right to undermine the state.
The latest developments in the case follow the 2015 deathbed confession of a former policeman who admitted to being one of the Brabant Killers.
